Sn—Pb alloys have continued to be used as solder since ancient times. They have advantages of low melting points and good solderability. For example, a eutectic Sn63%-Pb37% solder, the most popular solder, has a melting point of 183° C.
The eutectic Sn—Pb solder has also been commonly used in soldering of electronic devices. Recently, surface mount technology (SMT) has been employed increasingly to mount electronic parts on a printed board due to the capability of SMT to reduce size, enhance packaging density and performance, and lower operating costs. In SMT, soldering is typically carried out by the reflow soldering method using a solder paste (also called a cream solder) which comprises a solder powder uniformly mixed with a soldering flux, particularly a rosin-based flux. In general, the reflow soldering method comprises feeding a solder paste onto a printed board by printing or dispensing (discharging through a dispenser), temporarily fixing chip-type electronic parts on the solder paste by use of the adhesion of the paste, and heating the printed board with the electronic parts thereon in a reflow furnace to allow the solder to melt, whereby the parts are fixed and electrically connected to the printed board.
Printed boards withdrawn from waste electronic devices by disassembly are frequently discarded by pulverizing in a shredder and burying in the ground. When rain of recent years which has been acidified (i.e., acid rain) contacts the pulverized printed boards buried in the ground, the lead (Pb) in the Sn—Pb solder may be dissolved out and contaminate underground water. If a human or animal continues to drink a lead-containing water for long years, there is the concern that lead is accumulated in its body to cause lead poisoning. Therefore, it has been recommended in the art to use a lead-free solder, which is completely free from lead, in soldering of electronic devices.
Lead-free solders should consists of element which are harmless to humans. For example, Cd should not be used, even though it has an effect of lowering a melting temperature. Lead-free solders which are promising at present are Sn-based alloys which comprise a major proportion of Sn and one or more alloying elements such as Ag, Cu, Bi, In, Sb, or Zn.
Among these, Ag-containing Sn-based solders (hereunder referred to as Sn—Ag based solders) such as an Sn—Ag alloy and an Sn—Ag—Cu alloy have the advantage of being easy to handle since they have relatively good wettability as a lead-free solder. However, Sn—Ag based, lead-free solders have a melting point on the order of 220° C., which is approximately 30-40° C. higher than that of a eutectic Sn—Pb alloy, so the working temperature (heating temperature in soldering) also becomes higher correspondingly. Therefore, they may not be suitable for use in soldering of some thermally sensitive electronic parts.
Zn-containing Sn-based solders (hereunder referred to as Sn—Zn based solders) are advantageous from the standpoints of safety and economy, since Zn is not only an element which is harmless and indispensable for the human body but is found underground in large amounts so that its cost is low compared to Ag, Cu, Bi, In, and the like. A typical alloy composition of an Sn—Zn lead-free solder is Sn-9Zn. This solder has a melting point of 199° C., which is approximately 20° C. lower than that of an Sn—Ag lead-free solder, so it also has the advantage that it can be used for soldering thermally sensitive electronic parts to which an Sn—Ag based, lead-free solder cannot be applied.
In a lead-free solder paste comprising a powder of an Sn-based, lead-free solder including an Sn—Ag and Sn—Zn based solder (such solder paste being hereunder referred to as Sn-based, lead-free solder paste) mixed with a rosin-based flux, a reaction between the solder powder with a constituent of the flux, particularly with an activator which is normally present in the rosin-based flux, may occur readily due to the high content of the active Sn metal in the solder powder, compared to a conventional solder paste prepared from an Sn—Pb solder powder and the same rosin-based flux. Such a reaction may result in a change in viscosity of the solder paste and cause the problem that the solder paste cannot satisfactorily be fed by printing or dispensing.
As a countermeasure, an approach can be employed, particularly in a solder paste of an Sn—Ag based solder, in which the viscosity change is controlled by decreasing the amount of an activator, e.g., an amine hydrohalide and organic acid, added to the rosin-based flux, thereby suppressing a reaction of the flux with the solder powder. However, since this approach weakens the activity of the flux, it may cause the formation of solder balls and adversely affect the wettability of the solder. Accordingly, there is a need to control the viscosity change of an Sn-based, lead-free solder paste without decreasing the amount of an activator in the flux.
In addition to the above-described problem, a solder paste comprising a powder of an Sn—Zn based, lead-free solder (hereunder referred to as an Sn—Zn based solder paste) has another problem. Zn is a metal which is susceptible to oxidation due to its high ionization tendency. Therefore, an oxide layer is formed on the surface of the solder powder which contacts air, and it makes the wettability of the solder powder poor. In particular, in an Sn—Zn based solder paste prepared by use of a rosin-based flux, the surface oxidation of the solder powder by a reaction with the flux becomes even more severe, so soldering defects including formation of voids due to extremely poor wettability of the solder and formation of solder balls may occur frequently.
In order to increase the wettability of the solder in the Sn—Zn based solder paste, it is conceivable to use a rosin-based flux containing an increased amount of an activator. However, as described above, the activator tends to react with the solder powder prior to soldering, and an increase in the amount of an activator may cause the viscosity of the solder paste to increase rapidly and interfere with the feeding of the solder paste by printing or dispensing.
In an Sn—Zn-based solder paste, an approach in which the surface of the Sn—Zn based solder powder is coated with a suitable material prior to mixing with the flux to prevent the solder powder from reacting with the flux and causing surface oxidation may be employed. As the coating material, a precious metal such as Au or Pd, an inorganic oxide formed from a hydrolyzable organosilicon compound or the like, or an organic substance such as an imidazole or a triazole can be used.
However, such coating adds to the manufacturing costs of a solder paste. Furthermore, some types and methods of coating may promote oxidation of solder powder during the coating operation, and the method is not necessarily effective at improving solder wettability or solderability.